Method for creating a fiber optic section having spatial grating strength perturbations

ABSTRACT

A method for creating nonuniform grating strengths within fiber optic includes exposing photosensitive fiber optic material to an optical source for various periods of time through a mask. As the fiber optic material is exposed, the grating strength of the fiber changes. By linearly exposing the fiber optic material to different durations of optical energy, nonuniform grating strengths are created within the fiber, thereby creating a fiber optic section having asymmetric grating strengths.

This invention was made with Government support under Agreement No.DAAL01-95-2-3505 awarded by the Department of the Army. The Governmenthas certain rights in this invention.

This is a division of application Ser. No. 09/064,464 filed Apr. 22,1998 now U.S. Pat. No. 6,005,877.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates generally to fiber lasers and, moreparticularly to a fiber laser having asymmetric output ports.

(b) Description of Related Art

Fiber lasers are used in a variety of applications. As with all lasers,a fiber laser generates coherent light wherein the amplitude,polarization, frequency or wavelength, and phase of the output laserlight can be controlled. In general, fiber lasers include an opticalpump source, two reflectors comprising the optical cavity of aresonator, and an active region within the cavity. Unlike other lasers,the cavity and active region of a fiber laser are formed in an opticalfiber. The fiber generally includes a doped glass core that acts as thelaser's active region. In operation, the pump is coupled, via one end ofthe resonator, to the doped-glass core active region. The ions in thedoped core are excited by the pump to generate light that is reflectedbetween the reflectors. At least one of the reflectors of the resonatoris partially reflective, thereby allowing a portion of the laser lightto escape the cavity as the laser output.

Erbium is commonly used as a dopant for fiber lasers. The doped opticalfiber is pumped with an optical source having a wavelength λ_(p). As thedoped optical fiber is pumped above a threshold level it lases, emittingcoherent optical energy at a wavelength λ_(s).

Fiber lasers have many uses in the communications industry includingtelecommunications or ether systems using coherent detection methods.Fiber lasers may also be used as local oscillators in commercial radarapplications. Recently, fiber lasers have been proposed for use inautomobile collision-avoidance systems.

Two configurations of fiber lasers known in the art are the distributedBragg-reflector (DBR) configuration and the distributed feedback (DFB)configuration. The DBR laser uses a DBR element and at least one otherreflector, which is typically a DBR, to provide the necessary opticalfeedback for the narrowband lasing process. The DBR laser requiresspectral alignment of the distinct DBR elements, which is difficult toachieve in a manufacturing situation. Alignment of the DBR elements istime consuming and expensive. A further disadvantage of the DBR laser isthat it generally requires a long cavity length (relative to a DFBlaser) to separate internal reflections that form in the cavity of thelaser. The relatively long cavity length is also required to obtainproper spacing of the allowed lasing frequencies (longitudinal modes)with respect to the spectral bandwidth of net gain of the laser relatedto the bandwidth of the DBR elements and ensure one lasing frequency.Long cavity lengths in high erbium concentration doped fibers can leadto increased noise bursts during laser operation. These noise bursts aredue to the groups of erbium ions that collect in a nonuniform mannerwithin the cavity.

In DBR fiber lasers, it is known to adjust the reflector elements sothat most of the output power flows from one end of the laser cavity.Typically only one of two output ports provides the necessary poweroutput from the laser, which lowers laser cavity loss and laserthreshold power for oscillation.

The distributed feedback (DFB) laser configuration uses a short cavity(relative to a DBR) because the grating reflector spans the length ofthe cavity, thereby distributing the optical feedback. The gratingreflector used is the same length as an individual reflector element inan analogous DBR configuration with identical frequency discrimination.Due to the shorter cavity length there are fewer erbium ion clusterswithin the cavity as compared to the longer cavity of a DBR laser.Therefore, there is a lower probability of noise bursts during laseroperation of a DFB fiber laser. The longitudinal mode spacing isapproximately the same as the bandwidth of the grating reflectivity,which leads to ease of single frequency laser operation. Anotheradvantage that the DFB laser has over the DBR laser is the eliminationof the need to spectrally align distinct reflectors because only asingle DBR reflector is used. Therefore, the DFB laser is easier tomanufacture than the DBR laser Asseh et al. discloses a DFB fiber laserin 10 cm Yb³⁻ DFB fibre laser with permanent phase shifted grating,Electronics Letters, Jun. 8th, 1995, Vol. 31, No. 12, p.969-70.

DFB fiber lasers are symmetric in nature, which precludes an asymmetricpower flow design. That is, it is currently available technology doesnot allow on to design a DFB fiber laser that is symmetric yet does notoutput the same power from both ends of the laser.

It can be readily appreciated that the DFB laser configuration providessignificant advantages over the DBR laser configuration. However, knownDFB fiber laser configurations cannot be designed for asynmetric powerflow, which is desirable in many control and communication situations.

SUMMARY OF THE INVENTION

The present invention is embodied in a distributed feedback (DFB) fiberlaser having a laser cavity, a high power output port, and a low poweroutput port. The length of the fiber laser cavity has a designed spatialvariation in grating strength.

The present invention is also embodied in a method for creating a fiberoptic section having a spatial variation in grating strength. The fiberoptic section is created using a photosensitive optical fiber, anoptical energy source, a mask between the photosensitive optical fiberand the optical energy source, and a shield between the optical energysource and the mask. The varying grating pattern is created by exposingthe photosensitive optical fiber with the optical energy source andmoving the shield linearly from a first position to a second position.

The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram representing an embodiment of the DFBfiber laser of the present invention and a graph of the grating strengthof the fiber as a function of position;

FIG. 2 is a schematic diagram of a digital optical communication systememploying a fiber laser embodying the present invention; and

FIG. 3 is a diagram showing a configuration that may be used in thefabrication process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is embodied in a distributed feedback (DFB) fiberoptic laser having asymmetrical output ports. Traditionally, thesymmetric nature of a standard DFB laser has precluded an asymmetricpower flow design. The present invention uses varying grating strengthas a function of axial position in the fiber laser to provide asymmetricoutput ports.

Referring now to FIG. 1, a DFB fiber optic laser 50 is shown. The laser50 includes a high power port 55 and a low power port 60. The laser 50includes a number of segments 65, wherein the perturbation in thegrating strength (i.e., the grating index or the index of refraction) ofthe segments increases with position from left to right. A graph 70showing grating index as a function of fiber position (x) is shown belowthe laser 50. As shown in the graph 70 the amplitude of grating indexperturbations increases from a low peak value at the high power port toa high peak value at the high power port. The grating indexperturbations in the fiber from the high power port 55 to the low powerport 60 result in different reflectivity from the high power port 55 tothe low power port 60. A high reflectivity results for light propagatingwithin the laser 50 in the direction of large perturbation of thegrating index. Conversely, the counter-propagating optical energy thatencounters small perturbations in the grating index sees a lowerreflectivity.

The variations in reflectivity from one end of the laser 50 to the othermake it possible to have different power output at the ports 55, 60 ofthe laser 50. For example, as optical energy propagates toward the lowpower port 60 it is reflected to a large extent. The energy that is notreflected at the low power port 60 is transmitted from the low powerport 60. As optical energy propagates toward the high power port 55,some of the energy is reflected and some of the energy is transmittedfrom the high power port 55.

The laser 50 of the present invention may be pumped through either thehigh power port 55 or the low power port 60, the grating index of thefiber does not affect the pump wavelength power. When sufficient pumpenergy is coupled to either port 55, 60 the laser 50 becomes active andoutputs an optical signal from both of the ports 55, 60. Table 1illustrates typical power output from the laser output ports 55, 60 asthe laser is pumped through one of the ports 55, 60. The empiricalresults as shown in Table 1 represent the variation in power outputbetween the high power port 55 and the low power port.

                  TABLE 1                                                         ______________________________________                                                                Power Level Measured                                  Pump Port Output Port Measured                                                                        (μW)                                               ______________________________________                                        Low       Low           10                                                    Low       High          1000                                                  High      Low           25                                                    High      High          525                                                   ______________________________________                                    

FIG. 2 is a schematic diagram of an optical communication transmitter 80using the fiber laser 50 of the present invention. The communicationsystem 80 generally includes a pump laser 90, a plurality of wavelengthdivision multiplexers (WDM) 95, 100, 110, 120, an optical isolator 105,a fiber amplifier 115, a polarization controller 125, and the fiberlaser 50 of the present invention. The communication system 80 alsogenerally includes an optical detector 130 and a feedback circuit 135that are used in a feedback configuration to control the pump laser 90in order to minimize amplitude noise in the fiber laser output power.

The pump laser 90 provides optical energy λ_(p) to the fiber laser lowpower port 60, via a first WDM. The WDM device is designed to allowlight at two wavelengths to be present at one port. However, only ligateat single wavelengths is present at each of the remaining two ports. Forexample, the port of the WDM 95 that is connected to fiber laser 50 hasboth the wavelength of the pump laser λ_(p). and the wavelength of thefiber laser output signal λ_(s). The WDM 95 port connected to the pumplaser has only the pump wavelength λ_(p) and the WDM 95 port connectedto the detector 130 has only the fiber laser output signal λ_(s).

As sufficient energy from the pump laser 90 reaches the fiber laser 50,the fiber laser 50 begins to lase emitting light at wavelength λ_(s).The majority of the fiber laser output signal and the pump signal arepassed out the high output port 55 to another WDM 100, which separatesthe two signals to two different ports. This strips the pump signal fromthe fiber laser signal. The fiber laser signal is passed to the opticalisolator 105, which minimizes back-reflections that may destabilize thefiber laser 50. From the optical isolator 105 the fiber laser signal ispassed to a WDM 110 that combines the fiber laser signal with the pumpsignal and couples both signals to the fiber amplifier 115. The fiberamplifier uses the pump signal to amplify the fiber laser signal. In oneEmbodiment the fiber amplifier 115 may be a Lycom™ product having partnumber R47PM02.

Another WDM 120 is used to strip the pump energy from the fiber laserenergy. The port of the WDM 120 having the energy from the fiber laseris coupled to the polarization controller 125, which properly polarizesthe energy for use by an optical intensity modulator 140. In oneembodiment the modulator 140 may be fabricated from lithium niobate andmay be purchased from Uniphase Telecommunications Products.

The energy from the fiber laser 50 that is not coupled out of the highoutput port 55 is coupled out of the low output port 60. The signal fromthe low output port 60 is coupled to the WDM 95 and back to the detector130. The detector 130 converts the optical signal into an electricalsignal that is coupled to the feedback circuit 135. The feedback circuit135 processes the electrical signals to derive a control signal relatedto the amplitude noise in the fiber. The signal is used to control theamplitude noise in the laser by adjusting the pump laser output. If thefiber laser 50 is not being pumped properly the feedback circuit 135adjusts the pump laser 90 to properly pump the fiber laser 50. Digitalcommunications using a system such as the one shown in FIG. 2 yield verylow bit error rates (≦10⁻¹²).

FIG. 3 is a diagram illustrating a configuration that may be used toproduce sections of optical fiber having the proper grating for creatingthe fiber laser of the present invention. The configuration includes alaser source 200, a phase mask 210, a section of photosensitive, laseractive optical fiber 220, and a laser shield 230.

The laser source 200 is preferably a KrF eximer laser that emits energyat a wavelength of 248 nanometers (nm). Energy from the laser source 200passes through the phase mask 210, which is designed to direct a largefraction of the energy into the +1 and -1 diffracted orders. These twooptical fields overlap in the vicinity of the phase mask to form anoptical interference pattern. The optical fiber 220, which isphotosensitive to the laser source 200, is placed within the region ofthe interference pattern and converts the intensity pattern into asimilar variation in the index of refraction. Due to this opticaldamaging effect an internal grating is formed within the optical fiber220. The longer that the optical fiber 220 is exposed to the energy fromthe laser sources 200, the more the local perturbation of the index ofrefraction of the fiber is increased.

To create a fiber that has a grating profile as shown in the graph 70 ofFIG. 1, the laser shield 230 is moved linearly across the interfacebetween the laser source 200 and the phase mask 210. As the laser shield230 blocks energy from the optical fiber 220 the optical damage to thefiber is stopped and the grating index perturbation of the fiber stopsincreasing. Therefore, as the laser shield 230 moves linearly fromblocking none of the laser energy from the fiber to blocking all laserenergy from the fiber, a varying grating strength is created.

Of course, it should be understood that a range of changes andmodifications can be made to the preferred embodiment described above.For example, rather than using a mask, a spot beam laser may be used toscan the length of the fiber at various speeds to create the gratingperturbations in the fiber. Alternatively, a fixed aperture laser may bemoved across the fiber and paused at various locations to create thegrating perturbations in the fiber. It is therefore intended that theforegoing detailed description be regarded as illustrative rather. thanlimiting and that it be understood that it is the following claims,including all equivalents, which are intended to define the scope ofthis invention.

What is claimed is:
 1. A method for creating a fiber optic sectionhaving a spatial variation in grating index perturbations, the methodcomprising the steps of:providing a photosensitive optical fibersensitive to optical energy of a first frequency; providing an opticalenergy source outputting optical energy at the first frequency, theoptical energy source is oriented in such a manner to project its outputenergy toward the photosensitive optical fiber; providing a mask betweenthe photosensitive optical fiber and the optical energy source;providing a shield between the optical energy source and the mask;exposing a portion of the photosensitive optical fiber with the opticalenergy source, the exposed portion having an axial center; and movingthe shield from a first position to a second position, so that when theshield is in the first position substantially none of the optical energyfrom the optical energy source is blocked from the photosensitiveoptical fiber by the shield and when the shield is in the secondposition substantially all of the optical energy from the optical energysource is blocked from the photosensitive optical fiber by the shield,to create grating index perturbations on a first side of the axialcenter of the exposed portion that are asymmetric with respect tograting index perturbations on a second side of the axial center of theexposed portion.
 2. The method of claim 1, wherein the photosensitiveoptical fiber is doped with erbium.
 3. The method of claim 1, whereinthe optical energy source comprises a KrF excimer laser.
 4. The methodof claim 1, wherein the shield movement is perpendicular to the opticalenergy source and parallel with the optical fiber.
 5. The method ofclaim 1, wherein the step of exposing comprises creating a peak gratingindex on the first side greater than a peak grating index on the secondside.